The present invention relates to methods for analyzing progressive damage to solid surfaces, and in particular, to damage arising from fretting fatigue.
The ASM Handbook on Friction, Lubrication, and Wear Technology describes fretting fatigue as the progressive damage to a solid surface that arises from fretting. Fretting is defined as a wear phenomenon occurring between two surfaces having oscillatory relative motion of small amplitude. The phrase crack nucleation, as used in this specification, refers to an operating time period to the occurrence of a crack formation in a part being analyzed or treated. This operating time parameter is typically a number of cycles. The symbol used herein for crack nucleation is (Ni).
The first detailed stress analysis of fretting fatigue appears to have been done by Mindlin (see Mindlin, R. D., 1949, xe2x80x9cCompliance of Elastic Bodies in Contactxe2x80x9d, ASME J. of Appl. Mech., Vol. 16, pp 259-268) and remains the predominant basis for addressing a variety of practical problems. In particular, Mindlin""s solution has been extensively cited in the literature on fretting fatigue in the context of developing structural life prediction models. Several models have been proposed, each involving one or more parameters assumed related to damage accumulation and failure due to fretting fatigue. Relative efficacies of several such models have been discussed by Szolwinski, et al (see Szolwinski, M. P., Harish, G., McVeigh, P. A., and Farris, T. N., 1999, xe2x80x9cExperimental Study of Fatigue Crack Nucleation in Aerospace Alloys with Emphasis on Life Predictionxe2x80x9d, ASTM STP 1321, Am. Soc. of Test. and Matls.). Some of the proposed models include consideration of stresses near the contact boundary. In these models, generally, the focus is on estimating and using stresses and strains at some fixed distance (d) from the contact boundary. Most models also necessitate assuming a value for coefficient of friction. To some extent, the necessity of choosing d and a constant coefficient of friction reduces objectivity in the application of these models.
What is needed is a better method for analyzing fretting fatigue.
The present invention provides a method for determining crack nucleation for a part subject to fretting fatigue. Fretting fatigue tests are performed on at least one test specimen having a material composition similar to the part. The results of the test are analyzed to determine a variation of stress intensity factor as a function of time which may be referred to as (Q{N}) for the test specimen. A relationship is then determined between (Q{N}) and crack nucleation (Ni) for the test specimen. A variation of stress intensity factor as a function of time (Q{N}) for the part is determined based on the operating conditions and loads of the part. The (Q{N}) for the part is then corresponded to the (Q{N}) for the test specimen to determine a crack nucleation value (Ni) for the part. In a preferred embodiment, a computer is programmed to conduct the analysis.